Valuable items such as negotiable instruments, art work, etc., are susceptible to theft and counterfeiting. With respect to documents in particular, the advancement of color copier technology has made it fairly easy to create a color copy of any document, including currency, using commonly available equipment.
One security device for verifying an article's authenticity is a digital watermark. Digital watermarks or similar signatures typically use semantic information associated with the protected article, such as alphanumeric characters, physical features, etc. or other related information such as ownership information. These watermarks or signatures are typically kept with or incorporated into, the item to be protected. For example, a watermark may be printed within the substrate of a negotiable instrument including information regarding the value and the originator of the instrument. As another example, digital watermarking techniques may be used for both still and video images.
Various digital watermarking techniques are known for both still and video images. For example, reference may be had to U.S. Pat. No. 6,343,138 B1, entitled “Security Documents with Hidden Digital Data”, issued Jan. 29, 2002. This patent discloses embedding a visible digital watermark into a video signal or a still image. Another example of use of digital watermarks with documents is disclosed in U.S. Pat. No. 6,243,480 B1, entitled “Digital authentication of analog documents,” issued Jun. 5, 2001.
There are many techniques that are used to embed additional information in a digital image. One example is disclosed in U.S. Pat. No. 6,359,985, entitled a “Procedure for marking binary coded data sets,” issued Mar. 19, 2002.
One technique involves separately coding an image and a watermark image on an article using a pseudo random number generator and a discrete cosine transform (DCT) to term coded blocks, one of the image to be watermarked and the other of the watermark itself. The DCT coefficients representing the coded watermark block and the coded image block are then added together to form a combined block to digitally watermark the image. For example, as disclosed in U.S. Pat. No. 6,037,984, entitled a “Method and Apparatus for Encoding a Watermark into a Digital Image or Image Sequence,” issued Mar. 14, 2000, an image or sequence of images is watermarked using a conventional DCT unit and quantizer by generating an array of quantized DCT coefficients and watermarking the array by selecting certain DCT coefficients and replacing them with zero values. In the aforementioned patent, the array is further processed by a watermark inserter that replaces the zero valued coefficients with predefined watermark coefficients, and the coefficients form a watermarked array of DCT coefficients that constitutes a watermarked image.
Other common and known techniques for providing authentication of articles is through the use of invisible markings. One method is by printing in inks that are clear under normal illumination but emit visible fluorescent wavelengths upon ultraviolet (UV) excitation. One common technique is to use invisible ink to print security marks or information.
For documents with visible markings, it is very easy to copy or scan a document into a PC using scanning and printing technologies. In some instances, there is no way to tell if a reproduction is an original document. Even if the original document is coded with a visible code, once scanned, the code could be reproduced on the counterfeit document.
By using an invisible code or message on the document, for example “Original document—Do not copy”, a scanner using visible light would not see the invisible ink. Another security feature of invisible codes recognizes that it may be difficult to reproduce the invisible ink for printing illegitmate copies. Typically, the formulation of such ink is difficult to duplicate.
Another patent, U.S. Pat. No. 6,269,169 B1, “Secure Document Reader And Method Therefor,” issued Jul. 31, 2001, discloses an apparatus and a method for reading documents.
Additional approaches to providing authentication use fluorescent markings and more sophisticated printing methods, such as printing with visible graphics. Of interest to the teaching of this invention is U.S. Pat. No. 6,373,965, “Apparatus And Methods For Authentication Using Partially Fluorescent Graphic Images And OCR Characters”, issued Apr. 16, 2002 discloses a system combining a source of ultraviolet light with apparatus for capturing and recognizing either graphic images or characters, or both. In this patent either a visible sub-image, or a fluorescent sub-image, or a combination image, may further contain steganographic (digital watermark) information that is encoded and inserted using conventional techniques. The digital watermark information may be similarly encoded and inserted into visible and/or fluorescent sub-portions and or recombined OCR characters.
Techniques for hiding information in an invisible fashion also include the use of various frequency bands in document security design. These optical techniques exploit the limitations of digital copiers and scanners by use of fine line printing patterns that use screen angle modulation of lines (SAM) and micro screen angle modulation techniques. For example, a protected document that includes background printed background matter and line pattern printed warning indicia is disclosed in U.S. Pat. No. 4,582,346, entitled “Document Security System,” issued on Apr. 15, 1986. U.S. Pat. No. 5,171,040, entitled “Copy-Invalidating Document” issued on Dec. 15, 1992 discloses printing a pattern of warning indicia that is merged by human sensory perception with its background, but is distinguished and reproduced by xerographic copying as a result of the employment of two different screen values for the indicia and the background within and beyond the reproductive capability of the copier machine. U.S. Pat. No. 5,297,815, entitled “Security Printed Documents” issued Mar. 29, 1994, discloses a security document that provides an indication of when copied by a photocopy machine through the use of a background pattern and warning matter interspersed with blank areas of a substrate to provide a distinct intermeshed pattern so the warning printed is indistinguishable by the human eye. As methods of detecting and defeating security devices, such as digital watermarks, have become more sophisticated, there is a growing need for advancements in markings that provide additional security.